I find the notion of May and Corbyn working together even more far fetched than my scenario, and I don't think Corbyn's presence would make the EU any more likely to make concessions.

In my scenario, Corbyn agrees to a referendum only if May agrees to a general election if her deal is voted down. The problem is would her MP's even agree to her deal to call a GE in that event, because it would be a case of Turkeys voting for Christmas. So Corbyn would need a legal guarantee of a GE if the referendum goes against May. Is it even possible to pass a compound bill in Parliament providing both for a referendum followed by a GE if the referendum is lost? Cause no Tory is going to vote for an early GE after a referendum is lost.